The iFrame is only good when you are on your local network, since the browser needs to be able to directly reach the server. Same issue if you try to frame an independent Guacamole docker without ingress, because it will have a local IP.
I can confirm that Guacamole Client from alexbelgium/hassio-addons works perfectly fine to RDP / VNC to connect to computers on the local network.
If people just want to reach web pages or browse the Internet from their home IP remotely, they can use this addon: Home Assistant Add-On: Firefox